Quasi-orthogonal space time block coding (STBC) is a method utilized in some diversity transmission systems utilized in the field of wireless communication. The appeal of quasi-orthogonal STBC is that it seeks to enable wireless communication systems to utilize advantages of diversity transmission at a transmitting station, while allowing simplified decoding techniques at a receiving station.
Diversity transmission enables a stream of data to be transmitted via a plurality of transmitting antennas. Diversity transmission systems are described by the number of transmitting antennas and the number of data streams. For example, a diversity transmission system, which utilizes four transmitting antennas to transmit data from a single data stream may be referred to as a 4×1 diversity transmission system. The data stream may comprise a sequence of data symbols referred to as data symbols. With quasi-orthogonal STBC, each of the plurality of transmitting antennas may transmit one of the data symbols, or a permutated version of the data symbols, at a given time instant. Each of the data symbols may be referred to as a codeword. A single codeword may be represented s(i,k), where i represents an index to a transmitting antenna, and k represents a time index to the data symbol within the data stream. For example, s(0,0) may represent a first data symbol within a data stream that is transmitted from a first transmitting antenna while s(1,1) may represent a second data symbol within the data stream that is transmitted by the second transmitting antenna, s(2,2) may represent a third data symbol within the data stream that is transmitted by the third transmitting antenna and s(3,3) may represent a fourth data symbol within the data stream that is transmitted by the fourth transmitting antenna. Thus, with STBC, at a given time instant, each of the transmitting antennas may transmit a data symbol that occurs at a distinct time instant within the data stream.
A variation of STBC is space frequency block coding (SFBC). With SFBC, codewords are formed by selecting frequency carriers, or tones, which contain portions of data carried in a data symbol. With SFBC diversity transmission, at a given time instant, each of the transmitting antennas may transmit a codeword, or permutated version of a codeword, which comprises a portion of the data contained within a given data symbol. Collectively, the plurality of transmitting antennas may simultaneously transmit codewords, which may be combined to reconstruct the data symbol. A single SFBC codeword may be represented by s(i,k), where i represents an index to a transmitting antenna, and k represents a group of tones. For example, s(0,0) may represent a codeword that comprises tones 0, 4, 8 and 16 within a data symbol, which is transmitted by a first transmitting antenna, while s(1,1) may represent a codeword that comprises tones 1, 5, 9 and 17 within the data symbol, which is transmitted by a second transmitting antenna, s(2,2) may represent a codeword that comprises tones 2, 6, 10 and 18 within the data symbol, which is transmitted by a third transmitting antenna and s(3,3) may represent a codeword that comprises tones 3, 7, 11 and 19 within the data symbol, which is transmitted by a fourth transmitting antenna. A single transmitted codeword may comprise a plurality of tones represented by the tone group index, k.
In the case of diversity transmission, with either STBC or SFBC, the transmitted signal may be modified as it travels across a communication medium to the receiving station. This signal-modifying property of the communication medium may be referred to as fading. Each of the signals transmitted by each of the plurality of transmitting antennas may experience differing amounts of fading as the signals travel through the communication medium. This variable fading characteristic may be represented by a transfer function matrix, H, which comprises a plurality of transfer function coefficients, hj, that represent the differing fading characteristics experienced by the transmitted signals.
The transmitted signals with either STBC or SFBC may be received at a receiving antenna located at a receiving station. The receiving station may process the received signals to determine estimated values for the codewords carried by the transmitted signals. However, the task of computing estimated values for the codewords may be computationally complex even when quasi-orthogonal STBC or SFBC are utilized to suppress off-diagonal crosstalk.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.